This invention relates to enclosures for magnetic disk memory storage units of the type used for computer systems. In particular this invention relates to an enclosure for the disk pack in a, so called, high density storage unit in which the disk pack contains multiple disks, such as 10 recording disks including a servo disk, turning at high speeds, such as 3600 revolutions per minute, and high track densities of around 400 tracks per inch of information on the disks.
In such applications thermal considerations are quite important for several reasons. First, from startup of the magnetic disk unit until the unit is thoroughly warmed up, operating characteristics cannot be allowed to change much from temperature effects or errors will occur. The best way to minimize change in operating characteristics is to reduce the temperature change from the startup period to the thoroughly warmed up state. Further, temperature differences between various disks and head arms on the disk pack unit cannot be allowed to occur relative to the servo disk and servo head arm because differences in characteristics of the units could cause a temperature induced displacement in the disk. Finally, the ultimate warmed up temperature of the unit must be limited in order to assure best operating conditions.
Heat can occur or be generated in three fashions in the shrouded disk area. Spindle bearing, or spindle heat and the longer time constant for warm-up of the larger spindle mass compared to that of the disk causes the two to three disks next to the spindle to heat up much slower than the rest of the disks. Secondly, heat can be generated by disk windage; and thirdly, by the variable windage associated with the heads and supporting arms as they move out over the disks. Windage is used here to refer to heat generated by air turbulence or movement with respect to an object causing air to surface friction. With one or two magnetic disks and a magnetic disk memory unit, some of these, so called, windage effects would be comparatively small. But, with as many as 10 or perhaps more disks in a single unit, the amount of rotating surface in contact with air increases dramatically along with the potential for producing heat. Similarly with a 10 unit stack there are a substantial number of head arms in the enclosure which are in the path of moving air, all of which create air turbulence.
Specifically, these various factors of heat sources or heat production inside the disk pack enclosure produce at least four identifiable types of thermal drift or temperature changes which can cause changes in operating characteristics. The first of these is referred to as "active drift" which is associated with the change in disk diameter and head arm length as related to arm carriage position. This refers to the heat generated by turbulence of the stationary head arms as they move in and out of the enclosure to bring the heads into operating relation with the different tracks on the stack of disks. The worst case in this type of drift is where the heads have been located on outer most tracks of a disk stack for a sufficient amount of time for the air temperature to have stabilized and then there is a call to actuate the head arms to inner most tracks thereby causing the head arms to be inserted considerably further into the enclosure thus increasing air turbulence and heat production. The additional windage load from the head arms elevates the pack area temperature and the diameter disk and head expansions must be identical for all disk and head relationships in the pack or otherwise a physical displacement will occur within the pack for the heads associated with different disks.
The next type of drift is related to a disk pack which has not been operating and has for example been on a storage shelf but is placed in a magnetic disk machine that has been operating and was merely stopped for reloading. This type of drift is referred to as "cold pack warm machine" drift and can occur over a period of about 20 minutes. Related to this of course is the third type of drift related to the temperature gradients caused when a cold pack is in a machine which has not been operating and both the disk pack and the magnetic disk unit are powered up and data processing is immediately commenced. This is "cold pack cold machine" drift. This type of drift can occur over a period as long as an hour and a half.
And finally, the fourth type of thermal drift is that related to the situation in which a warmed up or temperature stabilized disk pack is in a warmed up operating unit but the cover of the unit has to be opened and closed for adjustments of the head assemblies commonly called "head alignment," or placement of head assemblies or for whatever purpose. This difference can take up to 30 minutes to compensate for the difference in temperature with the cover on or off in a unit for only a short time.
Prior art dealing with magnetic disk pack enclosures concentrates primarily on air purification within the disk pack or cleanliness of the disk pack surfaces and does not show concern with problems relating to non-symetric heat build up and rate of heat build up in the disk pack having numerous disks. Such prior art for example does not show concern for minimizing thermal drift within the disk pack in order to minimize operating tolerance requirements for the unit. Such prior art is illustrated in the U.S. Pat. Nos. 3,710,540; 3,731,291; and 3,839,734.